In the production of organic acids, such as acetic acid, lactic acid, etc., by fermentation it is usually required to neutralize the fermentation as it proceeds so that the pH does not fall too far and inhibit the fermentation organism. Many fermentations operate optimally near neutral pH. This pH control is usually carried out by the addition of a base such as NaOH or Ca(OH)2 to the fermentor. This means that the product of the fermentation is a dilute salt, such as sodium acetate or calcium acetate, of the organic acid, not the free acid itself. Therefore, if it is desired to recover the free acid, it is necessary to convert the salt back to the acid and separate the acid from the dilute aqueous broth.
Many methods have been proposed to address this problem. Among the simplest methods is the addition of a strong mineral acid, such as sulfuric acid, to the broth containing the organic acid salt. Because the sulfuric acid is a much stronger acid than the organic acids, it shifts the ionic equilibrium so that essentially all of the organic acid salt is converted to the free acid. However, the strong acid is itself simultaneously converted to a salt. If the salt is not useful it can be disposed of, but this is often an economic and environmental burden since the byproduct salt is produced in an equal molar amount as the organic acid.
Other methods have been proposed to recover the organic acid from the dilute salt solution. One of the more interesting is the use of an amine to convert the alkaline metal salt to an organic salt. For example, Urbas, U.S. Pat. No. 4,405,717, incorporated herein by reference in its entirety, describes the use of tributyl amine (TBA) and CO2 to convert a dilute calcium salt to an insoluble CaCO3 and a water-soluble organic complex of TBA and acetic acid at very high yield. Urbas suggests the extraction of the TBA acid complex from the dilute aqueous solution and then the concentration and “cracking” or thermal decomposition of the recovered organic complex to regenerate the TBA and the acetic acid. However, the thermal cracking of the concentrated complex leads to the creation of intractable byproducts of the TBA such as quaternary salts and “tars.” The subsequent loss of TBA is an operational and economic burden.
Verser et al (U.S. Patent Publication No. 2005/0256337), incorporated herein by reference in its entirety, describe the recovery of the acid from the extracted TBA acid complex by forming its ester directly from the extract.
A number of processes have been proposed for extraction and regeneration of amine complexes. King, et al., U.S. Pat. No. 5,412,126, and Baniel, U.S. Pat. No. 4,275,234, describe extraction and regeneration methods. Koga et al. U.S. Pat. No. 4,353,784 extracts with high boiling tertiary amine and distills the acid, similar to King. Wise et al. U.S. Pat. No. 5,068,188 extract with amine and back extract with limestone to make alkaline earth acetate, but this does not regenerate a free acid. Thomas et al published U.S. Patent Application 2006/0024801 A1 where they acidify with a low molecular amine, concentrate by evaporation, replace the low molecular amine with a high molecular amine in a distillation column and distill the acid from the high molecular amine. Datta et al. U.S. Pat. No. 5,723,639 focus mainly on ammonium salt but also claims amine-containing salts in which a light alcohol is added to the dilute salt; the mixture is heated in the presence of a catalyst and subjected to pervaporation with hydrophilic membrane.
Some of these methods depend upon back extraction into water at a different temperature, distillation of the acid, or distillation of the amine. The acid can also be back extracted into an aqueous base solution as in Bailey et al., U.S. Pat. No. 4,771,001.
None of these methods, however, have provided a simple low energy way to recover the dilute acid salt and produce the free organic acid.